The present invention relates to a fuel injection control device and method for an engine having a port injection valve and a direct injection valve.
In an engine, in order to control the air-fuel ratio of air-fuel mixture burned in a cylinder to a target air-fuel ratio, it is only necessary to determine a fuel supply amount such that the ratio of the fuel supply amount to the amount of air introduced into the cylinder (cylinder inflow air amount) becomes the reciprocal of the target air-fuel ratio. However, there are variations in the output characteristics of the air flow meter used to calculate the cylinder inflow air amount and in the injection characteristics of the fuel injection valves. Thus, mere determination of the fuel supply amount based on the cylinder inflow air amount calculated based on the output of the air flow meter will result in deviation of the air-fuel ratio from the target air-fuel ratio.
Such deviation of the air-fuel ratio can be corrected by air-fuel ratio feedback control that corrects the fuel supply amount in accordance with the difference of the air-fuel ratio with respect to the target air-fuel ratio. Further, the responsiveness of the air-fuel ratio feedback control can be improved by obtaining the deviation of the air-fuel ratio from the result of the air-fuel ratio feedback control and learning the deviation as an air-fuel ratio learning value, and reflecting the air-fuel ratio learning value in the air-fuel ratio feedback control. Variations of the air-fuel ratio may show different tendencies depending on the operating state of the engine. Therefore, the learning of the air-fuel ratio learning value is desirably executed separately for each of learning regions divided according to the operation regions of the engine.
Some engines have two types of fuel injection valves: a port injection valve, which injects fuel into the intake port, and a direct injection valve, which injects fuel into the cylinder. In this type of engine, the injection distribution control is executed in which the ratio of the fuel injection amounts from the two types of fuel injection valves is varied depending on the operating state of the engine. Since the port injection valve and the direct injection valve of such an engine have different tendencies in variations of the injection characteristics, the learning of the air-fuel ratio learning value is also preferably executed separately for each type of the fuel injection valves. The learning of the air-fuel ratio learning value for each type of the fuel injection valves can be executed by forcibly executing fuel injection only from one of the fuel injection valves.
Japanese Laid-Open Patent Publication No. 2005-307756 discloses a fuel injection control device. In a learning region in which neither learning of an air-fuel ratio learning value for the port injection nor learning of an air-fuel ratio learning value for the direct injection valve has been completed, the fuel-injection control device preferentially executes the learning of the air-fuel ratio learning value for the fuel injection that is set to have a greater fuel injection amount ratio in the setting of the injection distribution ratio in that learning region. During the injection distribution control, the deviation of the injection characteristics affects the air-fuel ratio to a greater extent in the fuel injection valve in which the ratio of the fuel injection amount is set to a great value than in the fuel injection valve in which the ratio is set to a small value. Therefore, if the completion of final learning occurs simultaneously, that is, if the learning of the air-fuel ratio learning values for both fuel injections is completed at the same time, the effects of the learning are obtained at an earlier stage when the learning of the air-fuel ratio learning value is executed in the order of the fuel injection having the larger ratio of the fuel injection amount and the fuel injection having the smaller ratio of the fuel injection amount than when the learning is executed in the reverse order.
As described above, in the fuel injection control device for an engine disclosed in the above publication, it is possible to cause the learning to take effect from an earlier stage on the condition that the final completion of learning occurs at the same time. However, when the learning of the air-fuel ratio learning value for the fuel injection of which the ratio of the fuel injection amount is set to a greater value is prioritized as described above, the final completion of learning may be delayed. This will delay the time at which the learning takes effect.